This invention relates to an endoprosthesis device for implantation within a body vessel, typically a blood vessel. More specifically, it relates to a tubular expandable stent of improved longitudinal flexibility.
Stents are placed or implanted within a blood vessel for treating stenoses, strictures or aneurysms therein. They are implanted to reinforce collapsing, partially occluded, weakened, or dilated sections of a blood vessel. They have also been implanted in the urinary tract and in bile ducts.
Typically, a stent will have an unexpanded (closed) diameter for placement and an expanded (opened) diameter after placement in the vessel or the duct. Some stents are self-expanding and some are expanded mechanically with radial outward force from within the stent, as by inflation of a balloon.
An example of the latter type is shown in U.S. Pat. No. 4,733,665 to Palmaz, which issued Mar. 29, 1988, and discloses a number of stent configurations for implantation with the aid of a catheter. The catheter includes an arrangement wherein a balloon inside the stent is inflated to expand the stent by plastically deforming it, after positioning it within a blood vessel.
A type of self-expanding stent is described in U.S. Pat. No. 4,503,569 to Dotter which issued Mar. 12, 1985, and discloses a shape memory stent which expands to an implanted configuration with a change in temperature. Other types of self-expanding stents not made of shape memory material are also known.
This invention is directed to stents of all these types when configured so as to be longitudinally flexible as described in detail hereinbelow. Flexibility is a desirable feature in a stent so as to conform to bends in a vessel. Such stents are known in the prior art. Examples are shown in U.S. Pat. No. 4,856,516 to Hillstead; U.S. Pat. No. 5,104,404 to Wolff; U.S. Pat. No. 4,994,071 to MacGregor; U.S. Pat. No. 5,102,417 to Palmaz; U.S. Pat. No. 5,195,984 to Schatz; U.S. Pat. No. 5,135,536 to Hillstead; U.S. Pat. No. 5,354,309 to Shepp-Pesch et al.; EPO Patent Application 0 540 290 A2 to Lau; EPO Patent Application No. 0 364 787 B1 to Schatz, and PCT Application WO 94/17754 (also identified as German Patent Application 43 03 181).
Generally speaking, these kinds of stents are articulated and are usually formed of a plurality of aligned, expandable, relatively inflexible, circular segments which are interconnected by flexible elements to form a generally tubular body which is capable of a degree of articulation or bending. Unfortunately, a problem with such stents is that binding, overlapping or interference can occur between adjacent segments on the inside of a bend due to the segments moving toward each other and into contact. Moreover, on the outside of a bend, the segments can move away from each other, leaving large gaps. This can lead to improper vessel support, vessel trauma, flow disturbance, kinking, balloon burst during expansion, and difficult recross for devices to be installed through already implanted devices and to unsupported regions of vessel.
A diamond configuration with diagonal connections between each and every diamond of each segment is also known but such closed configurations lack flexibility.
Such stents also suffer from the problem of shortening upon radial expansion. As the stent expands radially, it contracts lengthwise.
It is an object of this invention to provide a stent with a distributed structure which is longitudinally flexible that avoids these problems and exhibits improved flexibility in the stent body segments thereof rather than in flexible joints between the segments. It is a further object to provide stents that exhibit a desired lengthening or a desired shortening on radial expansion as well as stents which exhibit substantially no shortening or lengthening on radial expansion.
It is a further object of the present invention to provide a stent formed of a series of interconnected flexible cells.
It should be noted that for the purposes of this invention, the phrase generally sinusoidal is intended to include waves characterized by sine and cosine functions as well as waves which are not rigorously characterized by those functions, but nevertheless resemble such waves. In a more general way, such waves include those which are characterized as having one or more peaks and troughs. As an example, a wave whose peaks and troughs are U shaped or bulbous is intended to be included. Also intended to be included, without limiting the definition, are waves which are more triangular in shape such as a saw-tooth wave or waves whose peaks and troughs are rectangular.
The present invention provides a radially expandable stent having first and second ends and a longitudinal axis. The stent comprises a plurality of spaced band-like elements forming a hollow cylinder. The band-like elements are arranged sequentially along the cylinder and each band-like element comprises one or more sub-elements having a generally serpentine configuration to provide continuous waves to each sub-element. The waves are characterized by a plurality of peaks and troughs taking a generally longitudinal direction along the cylinder such that the waves in the sub-elements open as the stent is expanded from a first diameter to a second diameter. Adjacent band-like elements in the stent are connected together by one or more links. Each link has at least one bend therein and terminates in first and second shanks. The first shank of each link emanates from a region of attachment between a peak and trough on a sub-element of a band-like element while the second shank of each link emanates from a region of attachment between a peak and trough on a sub-element of an adjacent band-like element. The first shanks attached to any given sub-element of a band-like element are spaced substantially one wavelength or more apart along the sub-element of a band-like element. Likewise, the second shanks attached to any given sub-element of a band-like element are spaced substantially one wavelength or more apart along the sub-element of the band-like element.
The present invention is also directed to a radially expandable stent comprising a plurality of spaced band-like elements forming a hollow cylinder. The band-like elements are arranged sequentially along the cylinder. Each band-like element has a generally serpentine configuration to provide continuous waves of generally sinusoidal character to each band-like element. The waves are characterized by a plurality of peaks and troughs taking a generally longitudinal direction along the cylinder, the peaks and troughs having a midpoint region midway between them, such that the waves in the band-like elements open as the stent is expanded from a first diameter to a second diameter. The stent further comprises one or more spaced generally longitudinal elements extending from the first end of the stent to the second end of the stent and having alternating peaks and troughs and longitudinal transition regions midway between adjacent peaks and troughs. Adjacent longitudinal elements are in phase with one another. Each generally longitudinal element intersects each band-like element in a region of intersection, which includes a region between a peak and a trough on a band-like element, and a transition region of a longitudinal element. Adjacent longitudinal elements intersect each band-like element at least one wavelength apart along the band-like element.
The present invention is further directed to an expandable stent which in expanded form comprises a plurality of flexible connected primary cells. Each primary cell comprises a first member having first and second ends extending in a direction generally perpendicular to the longitudinal axis of the stent and having a serpentine shape. The first members each have one peak and one trough, the peak and trough taking a generally longitudinal direction along the stent. Each primary cell further comprises a second member having first and second ends extending in a direction generally perpendicular to the longitudinal axis, and having a serpentine shape. The second members have one peak and one trough, the peak and trough taking a generally longitudinal direction along the stent. The second member is situated generally opposite the first member and is optionally out of phase with the first member. The primary cells also comprise a first link having a first end and a second end, and extending in a generally longitudinal direction. The first link has at least one bend therein and is disposed between the first end of the first member and the first end of the second member. The first end of the first link is attached to the first end of the first member and the second end of the first link is attached to the first end of the second member. Finally, each primary cell comprises a second link having a first end and a second end, extending in a generally longitudinal direction. The second link has at least one bend therein and is disposed between the second end of the first member and the second end of the second member. The first end of the second link is attached to second end of the first member and the second end of the second link is attached to the second end of the second member. The second link is in phase with the first link. The primary cells are arranged in one or more primary bands and adjacent primary bands are interconnected. Primary cells in adjacent bands may optionally be offset relative to one another along the bands.
Optionally, the stent may further comprise secondary bands comprised of secondary cells, the secondary bands alternating with the primary bands.